Known processes for preparing an optically active .beta.-hydroxyketone (an optically active alcohol) through a catalytic asymmetrical aldol reaction include (1) reaction between an aldehyde and a silyl-enol ether in the presence of a binaphthol-oxotitanium complex (see T. Mukaiyama, et al., Chem. Letter., pp. 1015-1018 (1990)), (2) reaction between an aldehyde and a silyl-enol ether in the presence of a boron complex derived from tartaric acid (see K. Furuta, et al., Syn. Lett., pp. 439-440 (1991)), (3) reaction between an aldehyde and a silyl-enol ether in the presence of a boron complex derived from menthone (see E. R. Parmee, et al., Tetrahedron Lett., Vol. 33, pp. 1729-1732 (1992)), and (4) reaction between an aldehyde and nitromethane in the presence of a binaphthol lanthanoid complex (see H. Sakai et al., J. Am. Chem. Soc., Vol. 114, pp. 4418-4420 (1992)). However, these conventional processes were unsatisfactory in terms of catalytic activity and optical purity (diastereo-selectivity or enantio-selectivity). Besides, preparation of the above-mentioned complex catalysts involves complicated operation.